1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus including the sheet feeding device.
2. Description of the Related Art
There is known a sheet feeding device that quickly feeds sheets one by one to an image forming apparatus or an image forming unit. FIG. 1 illustrates a schematic configuration of the sheet feeding device.
The sheet feeding device of FIG. 1 blows air from a lift-blowing unit 1 toward front ends of a plurality of sheets stacked on a tray, and lifts the sheet up to a height of a suction belt (sheet absorbing unit) 2 by the wind. Then, the suction belt 2 absorbs one sheet at the top of the sheets by operation of an air suctioning unit 30. Here, the number of sheets absorbed onto the suction belt 2 may not be one. That is, a plurality of sheets adheres to each other and may together be absorbed by the air suctioning unit 30 sometimes. Thus, air is blown from a handling-blowing unit 24 provided in each side fence (sheet side end regulating member) 23 toward the side surfaces of the sheets so that the sheets absorbed onto the suction belt 2 can be handled and thus only one sheet may be absorbed onto the suction belt 2. Then, the sheet is transported to an image forming unit by the suction belt 2 so that an image is formed thereon.
A sheet blocking member 7 is disposed between the lift-blowing unit 1 and the sheet, and prevents the sheets except for the uppermost sheet from being transported. Further, in order to maintain a constant distance h between the suction belt 2 and the position of the uppermost surface of the sheets decreased due to feeding of the sheets, a detection unit 3 is provided to detect the height of the sheets while being in contact with the uppermost surface of the sheets. The detection unit 3 includes an actuator 4 and a sensor 5 that detects the motion of the actuator. When the actuator 4 swings due to the sheets decreasing in number, the movement amount of the actuator is detected by a photo sensor or the like, and a bottom plate 6 is elevated by an elevation unit or the like on the basis of a signal generated from the sensor, so that the distance is adjusted.
The sheets are evenly aligned at the front end portions thereof on the sheet feeding tray so as to fall within the sheet size. As illustrated in FIG. 1, the attachment position of the actuator 4 is set to the vicinity of the rear end of the sheet so as not to be easily affected by the wind blown from the blower.
Transportation rollers 8 are disposed on the downstream side of the suction belt 2, and transport the sheet arriving at the rollers. The transportation force of the transportation rollers 8 is set to be larger than that of the suction belt 2. Further, a sheet feeding sensor 9 is provided on the downstream side of the transportation rollers 8 to detect whether the sheet arrives at the position of the sensor.
Next, a sheet feeding operation of the existing sheet feeding device will be described in accordance with its procedure.
(1) When a sheet feeding command is sent from an image forming apparatus body, as illustrated in FIG. 2, the lift-blowing unit 1 and the handling-blowing unit 24 operate to blow air to the end of the sheet. At the same time, the suction belt 2 starts air suction. Accordingly, an uppermost sheet P1 is lifted, and the uppermost sheet P1 is absorbed onto the suction belt as illustrated in FIG. 2.
(2) The suction belt 2 and the transportation rollers 8 start to be driven, so that the sheet P1 is transported (FIG. 3).
(3) The driving operation of the suction belt 2 stops after the sheet P1 arrives at the sheet feeding sensor 9 (FIG. 4). The transportation rollers 8 continuously transport the sheet P1 while the suction belt 2 is stopped.
(4) A next sheet P2 is lifted and absorbed onto the suction belt by air right after the sheet P1 exits a suction area (FIG. 5).
(5) The driving operation of the suction belt 2 is resumed in accordance with the sheet feeding interval that is set, so that the sheet P2 is fed.
(6) Subsequently, the sheets stacked on the tray are sequentially transported by repeating the above-described procedure from (2) to (5).
In the above-described sheet feeding operation, the volumes of air of the lift-blowing unit 1, the handling-blowing unit 24, and the air suctioning unit 30 are not described. However, if the volume of air is fixed to a certain value, the lift amount or the handling state of the sheet changes in accordance with the thickness, the weight, or the size of the stacked sheets.
For example, when the lift amount of the sheet is small, feeding failure occurs. On the contrary, when the sheet is lifted too much, the sheets adhere to each other, so that a double feeding occurs. Further, when the force of the air suctioning unit 30 is small, the sheet is not satisfactorily transported, which also causes feeding failure.
For this reason, the volume of air suitable for the stacked sheets is set in advance in order to appropriately feed the sheet, and the volume of air is automatically set when a user selects the type of sheet to be fed. Then, the volume of air is adjusted by the value of the duty of the blower. For example, the lift-blowing unit 1, the handling-blowing unit 24, and the air suctioning unit 30 are set as described in the table below.
Lift-blowing unitHandling-blowing unitSheetAir suctioning unitA+30 [%]+20 [%]+40 [%]B+10 [%]−10 [%]+30 [%]C−20 [%]−40 [%]+20 [%]
As for the duty of the blower, 0[%] indicates a state where the blower does not rotate at all, and 100[%] indicates a state where the blower rotates at its full power. Here, the sheets are simply described as A, B, and C, but for example, the sheets may be described by a relationship as below in accordance with the thickness or the size thereof.
Sheet thickness: A>B>C
Sheet size: A>B>C
However, even when each blower is set to have a rotation speed suitable for blowing the desired volume of air for each type of sheets, an appropriate rotation speed may not be reliably obtained due to an imbalance of the blower or a variation with time in the characteristics of the blower.
In this case, for example, when the sheet is thin or small, the rotation speed of the blower serving as the handling-blowing unit becomes faster than a predetermined rotation speed, so the sheets are not reliably handled. This may cause double feeding or feeding failure. Further, when the sheet is thick or large, the rotation speed of the blower serving as the handling-blowing unit becomes slower than a predetermined rotation speed, so that air does not appropriately flow between the sheets. This may cause a double feeding or feeding failure.
The above conventional technique is for example related to Japanese Patent Application Laid-open No. 2007-045630.